The production of polycrystalline silicon for use in semiconductor devices is normally effected by the hydrogen reduction of various chlorosilanes at elevated temperatures. Because of the stringent impurity limitations for semiconductor silicon, the starting materials should be of the highest possible purity. Boron impurities in semiconductor silicon are generally the most troublesome since boron in silicon has a distribution coefficient approaching unity and therefore cannot normally be removed from silicon metal by conventional zone-refining techniques. Unfortunately, the boron-containing impurities are also difficult to remove from chlorosilanes.
There have been numerous attempts in the art to purify chlorosilanes in order to produce better quality semiconductor silicon. These purification techniques include, for example, repetitive distillation, water treatment (see for example U.S. Pat. No. 3,540,861 issued Nov. 17, 1970), and the use of adsorbants to remove the boron-containing and, possibly, other impurities. U.S. Pat. No. 4,112,057 (issued Sept. 5, 1978) teaches that chlorosilanes contaminated by boron-containing impurities can be purified by treatment of the liquid chlorosilane with an effective amount of a hydrated metal oxide or a hydrated silicate containing 3 to 8% by weight water and then distilling the treated chlorosilane at a temperature of about 3.degree. to 15.degree. C. above its boiling point at atmospheric pressure. Water is required to be added periodically to the adsorbant of U.S. Pat. No. 4,112,057 to keep the water content in the 3-8% range. Silica was employed as the adsorbant in this process but only liquid phase adsorption was disclosed in U.S. Pat. No. 4,112,057.
U.S. Pat. No. 3,071,444 (issued Jan. 1, 1963) teaches that chlorosilanes can be purified by passage of the liquid chlorosilane through a bed of various adsorbents including silica. For the silica to be effective, however, it was necessary to preactivate the material at about 270.degree. C. or more in air for extended periods of time. Additionally, long contact times between the chlorosilane and the adsorbent were used. U.S. Pat. No. 3,071,444 did not disclose the purification of chlorosilanes by passage of the chlorosilane in the vapor phase through any of the disclosed adsorbents.
Adsorption techniques employing ion-exchange resins have also been used to purify chlorosilanes (U.S. Pat. No. 2,877,097 issued Mar. 10, 1959) as well as organochlorosilanes (U.S. Pat. No. 3,414,603 issued Dec. 3, 1968). Both patents teach that adsorption of impurities (including boron) from the chlorosilanes by passage through ion-exchange resins was independent of the state (liquid or vapor phase) of the chlorosilanes. In other words, U.S. Pat. Nos. 2,877,097 and 3,414,603 teach that liquid phase adsorption and vapor phase adsorption of impurities from various chlorosilanes using ion-exchange resins are equivalent processes.
One object of this present invention is to provide a process by which chlorosilanes can be more easily purified. Another object is to provide a process by which boron-containing impurities can be more easily removed from chlorosilanes. Still another object is to provide a process for providing trichlorosilane, substantially free of boron-containing impurities, which is suitable for use in the production of semiconductor silicon. Still other objects of the present invention will be apparent to those skilled in the art upon consideration of this specification.